Fix use of last-state as starting-solution in Simulation.solve()

[jsbrittain]

Description

When a starting_solution was provided to Solution.solve(), the method attempted to pass a sub-field of the starting_solution to make_cycle_solution(), instead of the required list of Solver objects.

Fixes #2788 ([Bug]: Solution last_state can't be used as a starting solution)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c78bed2) 99.69% compared to head (ac47b03) 99.69%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2822   +/-   ##
========================================
  Coverage    99.69%   99.69%           
========================================
  Files          272      272           
  Lines        19035    19035           
========================================
  Hits         18976    18976           
  Misses          59       59           

Impacted Files	Coverage Δ
pybamm/simulation.py	100.00% <ø> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[martinjrobins]

Thanks @jsbrittain, happy to merge this